A major wiring method of a printed circuit board which is frequently used in electric products includes a method for etching foil of metal such as aluminum and copper. According to the conventional method, since materials are not the least lost in parts removed by etching, the method is not preferable from the viewpoint of effective utilization of the materials.
Further, since waste liquid, and the like, are generated due to etching, this method loads environment greatly. In recent years, from the viewpoint of resource saving and environment measure, wiring forming according to another method has been extensively investigated.
Among the new wiring forming technologies investigated, “printed electronics,” in which an existing printing technology is applied to form a wiring and a conductive film, have been expected to easily obtain those desired in a large amount, and therefore particularly have attracted attention.
As remarkable applications of the printed electronics, a print CPU, a print light, a print tag, an all-print display, a sensor, a print circuit board, an organic solar battery, an electronic book, a nanoimprint LED, a liquid-crystal display, a PDP display, a print memory, and RF-ID have been investigated. The scope of applicability is very wide.
The feasibility of such printed electronics largely depends on a metal component expressing conductivity. Therefore, in order to further promote the printed electronics technology, a metal particle which is a conductive particle has been widely investigated. From the viewpoint of a fine-wiring field which has great expectations of the print method and low-temperature sintering property, in particular, a metal nanoparticle having a nano-order particle diameter has been widely investigated (see Patent Documents 1 and 2).
When the metal has nano-size, it has been known that it has properties which greatly differ from physical properties in a bulk state. Since this nanosized particle has very high activity, the particle is unstable as it is. For this reason, as the nanoparticle, a nanoparticle having a covering layer mainly including organic matter such as a surfactant on the surface has been usually provided. As the composition, a composition, in which metal nanoparticles covered with the surfactant are dispersed in an organic solvent, has been mainly provided.
The surface of the metal nanoparticle is covered with organic matter such as a surfactant, as described above, to prevent sintering and aggregation of the particles. Use of a surfactant having a long chain prevents sintering and aggregation of the particles and is possible to ensure independency and storage stability of the particles in the liquid. Even if the metal is a nanosized particle, when a surfactant having a high molecular weight is configured around the particle, it is necessary to perform treatment at a high temperature for a time as long as 30 minutes to 1 hour to remove or degrade the surfactant on the surface of the particle during formulation of a metal film. This makes it difficult to apply the metal nanoparticle to a low-cost heat-labile wiring board, and therefore the probability of application using the metal nanoparticles may be reduced. Further, from the viewpoint of saving energy, this is inappropriate.
The metal nanoparticles are often dispersed in an organic solvent such as decane and terpineol. Since the organic solvent may cause environmental contamination, attention has to be paid to disposal. Since an organic component of the evaporated organic solvent diffuses easily, when a large amount of the organic solvent is used for treatment, it is necessary to install a local exhaust device. Of cause, it may damage our bodies. For this reason, from the viewpoint of environment and operation, it is desirable that a dispersion medium, in which the organic solvent is not a main component, be used.
When the application of the metal nanoparticles is a conductive material, the metal species of the metal nanoparticles is most suitably silver from the viewpoint of low resistance of metal, high acid resistance, low melting point and easiness of sintering, and basis metal price.
Based on the above knowledge, the present inventors have developed a technology for metal nanoparticles which has low-temperature sintering property and can form a metal film in a short time, and disclosed the content of the technology in the foregone application (see Patent Document 3).